Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes. Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” refer to manifolds with higher and lower pressures relative to each other. The pressure difference between the high and low pressure manifolds, and the absolute values of the pressure in the high and low pressure manifolds will depend on the application. A fluid working machine may have more than one low pressure manifold and/or more than one high pressure manifold.
Fluid working machines are known which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine. The net displacement of fluid also determines the torque applied to the fluid working machine's shaft. For example, EP 0 361 927 disclosed a method of controlling the net throughput of fluid (and therefore the torque) through a multi-chamber pump by opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to either displace a predetermined fixed volume of fluid or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net torque of the pump to be matched dynamically to demand. EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes. EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand. Such machines are called synthetically commutated fluid working machines, including the type known as a Digital Displacement pump/motor (Digital Displacement is a trade mark of Artemis Intelligent Power Limited).
Such fluid working machines are particularly useful in transmission systems, especially those for vehicles, and especially so-called ‘hybrid’ vehicles. US 2006/0118346 and WO 2006/055978 disclosed a number of layouts for transmissions incorporating synthetically commutated fluid working machines and also incorporating one or more fluid accumulators for energy storage. These transmissions are efficient because they can recover kinetic energy when the vehicle slows, then use the energy to accelerate the vehicle again some time later. WO 2008/012558 further disclosed a transmission and method of operation that requires only one high pressure side and one low pressure side, as well as eliminating the need for a precharge pump on the low pressure side, by operating in some modes directly from a reservoir at atmospheric pressure.
GB 2,430,246 (Stein) and EP 08164003.9 (Stein) both disclose two-stage valve assemblies which are suitable for regulating the supply of fluid from a high-pressure manifold to a working chamber of a synthetically commutated fluid working machine. The valve assemblies comprise a primary valve, a secondary valve, an electromagnet and an armature (referred to as a moving pole). The primary valve comprises a face-seating primary valve member and a primary valve seat. The secondary valve is integral to the primary valve and includes a secondary valve member which is moveable between a sealing position and an open position in which a path is provided through the secondary valve for fluid to flow between opposite sides of the primary valve member to reduce the pressure difference across the primary valve member. Thus, the secondary valve, which has a much smaller surface area than the primary valve, can be opened even when there is a substantial pressure difference across the primary valve member. The working chamber is effectively a closed volume, and so fluid can flow through the secondary valve to equalise the pressure on either side of the primary valve member, thereby facilitating the opening of the primary valve.
One problem of the two-stage valve assemblies is that the pressure in the working chamber must be made sufficiently high for the primary valve to be open, because of the limited force available from the armature. The length of time this takes depends on a number of variables such as high pressure manifold pressure, fluid temperature and working chamber leakage. Because of the uncertainty in these and other parameters, the opening operation of the primary valve has been found to be not reliable in some circumstances, causing the machine to operate incorrectly.
It is an object of the invention to provide an improved method of operating a fluid-working machine incorporating a two-stage valve assembly according to the prior art, so as to improve the reliability of the primary valve opening.
A further problem with fluid working machines operated by valve assemblies when they are applied to transmission systems is that the rapid application of pressure within the working chamber(s) due to the valve assembly opening leads to a sudden uncontrollable shaft rotation, where the shaft of the fluid working machine is connected to a typical load, especially one with the non-linear compliance known as ‘hysteresis’ or ‘backlash’, such as a transmission. The shaft may move too rapidly and uncontrollably in one direction, creating noise, excessive wear, mechanical fatigue, shock and discomfort for example.
It is therefore a further object of the invention to provide an improved method of operating a fluid working machine, incorporating a valve assembly, with a compliant load, for example a transmission such as a vehicle transmission, so as to control or limit the initial movement of the shaft when the valve assembly is actuated.
A further problem with transmission systems incorporating fluid working machines operated by valve assemblies is that the previously known methods of operating fluid working machines require the transmission system to adjust the pressure of the high pressure fluid source to accurately control the output torque at low or zero rotational speed, because it is only possible to have working chambers which are either fully-enabled or not enabled at all. Adjusting the fluid source pressure may not be possible, especially when a fluid accumulator is more or less directly connected to the fluid working machine, or may require additional components or energy transformations, increasing system cost or decreasing energy efficiency.
It is therefore a further object of the invention to provide an improved method of operating a fluid working machine incorporating a valve assembly, so as to control the torque applied to the shaft of the fluid working machine, at least when rotating at a slow speed.